The following test program generates extra use of a temporary register for the
return value. I have narrowed the cause of the poor code generation to the
addition of the pos = file->f_pos; line -- removing it causes the extra
temporary register to disappear (and saves several unnecessary mov
instructions).
/* compile with gcc -O2 -fomit-frame-pointer -fno-asynchronous-unwind-tables
-S -o fs/test-sys-read.s fs/test-sys-read.c && cat fs/test-sys-read.s
*/
struct file {
long f_pos;
};
struct file *fget_light(unsigned int fd);
long vfs_read(struct file *, char *, unsigned long, long *);
static inline int IS_ERR(const void *ptr)
{
return (unsigned long)ptr > (unsigned long)-1000L;
}
long sys_read(unsigned int fd, char *buf, unsigned long count)
{
struct file *file;
long ret, pos;
file = fget_light(fd);
if (IS_ERR(file)) {
pos = file->f_pos; // comment this line to get better code
ret = vfs_read(file, buf, count, &pos);
} else
ret = (long)file;
return ret;
}
--
Summary: code generation -- extra instructions generated
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: bcrl at kvack dot org
GCC build triplet: x86_64-unknown-linux
GCC host triplet: x86_64-unknown-linux
GCC target triplet: x86_64-unknown-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25225
